The present invention relates generally to electroslag refining, and, more specifically, to electroslag refining of superalloys.
Electroslag refining (ESR) is a process used to melt and refine a wide range of alloys for removing various impurities therefrom. U.S. Pat. No. 5,160,532--Benz et al. discloses a basic electroslag refining apparatus over which the present invention is an improvement. Typical superalloys which may be effectively refined using electroslag refining include those based on nickel, cobalt, zirconium, titanium, or iron, for example. The initial, unrefined alloys are typically provided in the form of an ingot which has various defects or impurities which are desired to be removed during the refining process to enhance metallurgical properties thereof including oxide cleanliness, for example.
In a conventional electroslag apparatus, the ingot is connected to a power supply and defines an electrode which is suitably suspended in a water cooled crucible containing a suitable slag corresponding with the specific alloy being refined. The slag is heated by passing an electric current from the electrode through the slag into the crucible, and is maintained at a suitable high temperature for melting the lower end of the ingot electrode. As the electrode melts, a refining action takes place with oxide inclusions in the ingot melt being exposed to the liquid slag and dissolved therein. Droplets of the ingot melt fall through the slag by gravity and are collected in a liquid melt pool at the bottom of the crucible. The slag, therefore, effectively removes various impurities from the melt to effect the refining thereof.
The refined melt may be extracted or drained from the crucible by a conventional segmented, cold-wall induction-heated guide (CIG). The refined melt extracted from the crucible in this manner provides an ideal liquid metal source for various solidification processes including, for example, powder atomization, spray deposition, investment casting, melt-spinning, strip casting, and slab casting.
In the exemplary electroslag apparatus introduced above, the crucible is conventionally water-cooled to form a solid slag skull on the surface thereof for bounding the liquid slag and preventing damage to the crucible itself as well as preventing contamination of the ingot melt from contact with the parent material of the crucible, which is typically copper. The bottom of the crucible typically includes a water-cooled, copper cold hearth in funnel form upon which a solid skull of the refined melt forms for maintaining the purity of the collected melt at the bottom of the crucible. The CIG defines a drain through the cold hearth and includes an upper funnel portion matching the funnel hearth, and a discharge drain tube or downspout therebelow, and is also typically made of copper, segmented, and water-cooled for also allowing the formation of a solid skull of the refined melt for maintaining the purity of the melt as it is extracted from the crucible.
An induction heater including a water-cooled electrical coil surrounds the guide tube for inductively heating the melt thereabove for controlling skull thickness. In this way, the thickness of the skull formed inside the drain may be controlled and suitably matched with melting of the ingot for obtaining a substantially steady state production of refined melt which is drained by gravity through the downspout.
In order to achieve steady state operation of the electroslag refining apparatus, the apparatus must be suitably started without introducing undesirable contamination or impurities. In a conventional cold start method, a solid starter plate is fixed into position at the bottom of the crucible and above the discharge guide tube.
Conventional slag in particulate form is deposited atop the starter plate around the electrode. An electrical current is passed through the electrode to the starter plate and then through the atmosphere to cause an electrical arc to jump therebetween. The heat from the arc melts the surrounding solid slag. When sufficient slag is melted, the electrode is lowered into the slag to extinguish the arc, at which time power to the electrode effects direct resistance heating of the slag pool for increasing its temperature.
The heated slag pool then continues to melt the tip of the electrode and the starter plate until a hole is melted through the starter plate and liquid metal begins to fill the crucible atop the guide tube. The induction heater is operated to initially allow the skull to plug the downspout to prevent premature draining of the melt until sufficient refining has occurred. The hole through the starter plate enlarges until it reaches the outer perimeter of the plate and the resulting refined metal and slag skulls line the crucible and the guide tube, and the refined melt pool is ready for draining The induction heater surrounding the downspout is then used to heat and melt the plug to commence and control draining. Steady state operation may begin when sufficient melt height is achieved in the crucible, and the rate of melting of the electrode and discharge flowrate from the guide tube are substantially equal.
However, induction heaters are expensive, especially when separate heaters are provided for the downspout and funnel portions of the CIG.
Accordingly, it is desired to provide a less expensive electroslag refining cold hearth and method of ESR starting.